1. Field of the Invention
The present invention relates to an RFID (Radio Frequency Identification) tag that performs information exchange with external equipment in a noncontact manner. Incidentally, among those skilled in the art related to the technical field of the present application, the “RFID tag” used in the specification of the present application may sometimes be called an “inlay for RFID tag” by regarding the “RFID tag” as an internal component member (inlay) for “RFID tag.” Or alternatively, in some cases, this “RFID tag” may be referred to as “a radio IC tag.” Also, a noncontact type IC card is included in this “RFID tag.”
2. Description of the Related Art
In recent years, there have been proposed various RFID tags that perform information exchange with external equipment represented by a reader/writer in a noncontact manner by use of radio waves. As one kind of this RFID tag, there has been proposed an RFID tag in which an antenna pattern for radio communication and an IC chip are mounted on a base sheet made of plastics or paper. A conceived mode of using an RFID tag of this type is such that the RFID tag is stuck to an article and the like and performs the identification of the article by exchanging information on the article with external equipment.
In such RFID tags, it has been proposed that a cover that covers a base sheet be provided in order to protect an antenna pattern or an IC chip.
FIGS. 1(A) and 1(B) are a front view and a side view, respectively, of a conventional RFID tag. The side view shown here is a drawing in which the internal structure is seen through from the side-surface side of the RFID tag. In this specification, drawings hereinafter called a side view are all similar drawings.
An RFID tag 1 shown in FIGS. 1(A) and 1(B) is constituted by an antenna pattern 3 provided on a base sheet 2, an IC chip 4 that is bonded to the base sheet 2 with an epoxy adhesive 7 and electrically connected to the antenna pattern 3 via a bump 5, and a cover sheet 6 bonded to the base sheet 2 in such a manner as to cover the antenna pattern 3 and the IC chip 4. The cover sheet 6 is usually formed from a material selected from among PET materials, polyester materials, polyolefin materials, polycarbonate materials, acrylic materials, etc.
This RFID tag 1 receives the energy of an electromagnetic field released by a reader/writer as electric energy by use of the antenna pattern 3 and the IC chip 4 is driven by the electric energy, whereby the communication action is realized.
In the RFID tag 1 constructed as described above, the height of the IC chip 4 portion is larger than the height of other portions. Therefore, when something rubs the cover sheet 6 of the RFID tag 1 and when the RFID tag 1 is used in such a manner as to be sandwiched between books, impacts and loads are concentrated on the IC chip 4 and this might cause troubles in the IC chip 4 and faults such as the exfoliation of the IC chip 4. Furthermore, there is also a possibility that the stretch or sag of the cover sheet 6 occurs near the IC chip 4, generating residual stresses, with the result that the cover sheet 6 might come off due to the residual stresses.
Compared to such RFID tags of typical structure, there have also been proposed RFID tags in which ideas for protecting IC chips are incorporated (refer to, for example, U.S. Pat. Nos. 6,100,804, 6,265,977, 6,147,604, 6,215,401 and 6,294,998). In RFID tags disclosed in these patent documents, an IC chip is embedded in a sealing member or an intermediate layer and the surface of the RFID tag is made flush, whereby the concentration of impacts and loads on the IC chip are avoided.
However, in such conventional RFID tags, the heat generated by the IC chip is captured and confined in due to the presence of the sealing member and the intermediate layer and this might cause malfunctions of the IC chip. Also, in the case of the RFID tag 1 of a typical construction as shown in FIGS. 1(A) and 1(B), the thermal conductivity of the cover sheet 6 is low, the thermal capacity of the IC chip 4 itself is also low and, besides, also between the IC chip 4 and the antenna pattern 3, the greater part except the portion of the bump 5 is embedded with an epoxy adhesive 7 and the thermal resistance by connection is large. For this reason, heat is apt to be captured and confined in the IC chip 4. Therefore, in a case where the RFID tag 1 is present near a reader/writer and subjected to a strong electromagnetic field, it might be thought that the temperature rises abruptly due to the heat generated in the IC chip 4. Such an abrupt temperature rise might cause malfunctions of the IC chip 4. Also, in a case where the temperature of an article to which the IC chip 4 is stuck is constantly a high temperature of 50° C. to 70° C., even when the RFID tag 1 is placed at a distance from a reader/writer, there is a possibility that a critical temperature for the stable operation in a transistor within the IC chip 4 and a critical temperature for long-term memory holding might be exceeded due to the heat generation from the IC chip 4.